TrueVoice.RTM. is a telephone network sound enhancement technology invented by AT&T Corp. TrueVoice provides AT&T customers with "closer," clearer sounding voice communications on telephone calls carried by the AT&T network. TrueVoice applies both a frequency independent gain (a constant gain across all frequencies in the telephone bandwidth) and a frequency selective gain (sometimes referred to as "pre-emphasis") to telephone connections. Technical features of TrueVoice are described in U.S. Pat. Nos. 5,195,132; 5,333,195; and 5,206,902(which are hereby incorporated by reference).
FIG. 1 is a diagram of a typical telephone network connection employing TrueVoice. It includes CPE 20 and 21 at two end points for a calling party and a called party, respectively, and analog and digital network components in between. CPE 20, 21 is, e.g., a conventional telephone. CPE 20, 21 is connected to a conventional hybrid converter 22, 23 at telephone central offices via local loops 31, 34. Each hybrid 22, 23 converts bi-directional signal transmission in a two-wire telephone line to two unidirectional signal paths (two wires each). The calling party's "transmit path" includes local loop 31, network paths 35 and 36 and local loop 34. This transmit path is also the called party's "receive path." The called party's transmit path includes local loop 34, network paths 38 and 37, and local loop 31. This transmit path is also the calling party's receive path. (Local loops 31 and 34 are common to both calling and called parties' transmit and receive paths.) The point of the network between paths 35 and 36/37 and 38 is said to be at 0 "TLP" (or transmission level point). This point may be conveniently used as a reference for gain or loss experienced at different points in the network.
Signals on the calling party transmit network path 35 are processed by a D/A-A/D converter 24, which is conventional equipment located at the calling part's local central office. (For clarity of presentation of the invention, conventional switches associated with the local central offices are not shown.) A long-distance telephone network switch, e.g., a No. 4 Electronic Switching System 26 (4ESS) in the AT&T Network, is connected to the D/A-A/D 24 converter of the local central office. The 4ESS 26 is then connected to a special automatic volume control filter (AVC) 30 which, as shown in FIG. 1, includes, for example, TrueVoice.RTM. elements 32 and 33. As a matter of general background, automatic volume control filters are conventional, for example, those described in U.S. Pat. Nos. 4,499,578 and 4,535,445, which are hereby incorporated by reference. TrueVoice element 33 applies the sound enhancement for speech signals spoken at CPE 20 for transmission to CPE 21. Element 32 applies echo cancellation to diminish an echo of speech signals (originally spoken at CPE 21) returning to CPE 21. As shown in FIG. 1, similar connections are used in network path 36 to deliver speech from the calling party. This path includes a -6 dB attenuator 29, intentionally inserted into the network, typically by the called party's Local Exchange Carrier (LEC), to further mitigate echo in a long distance connection (it is not needed in a local connection). The Figure further illustrates similar connections for network path 38 (which is like network path 35) and network path 37 (which is like network path 36). (Although much of the discussion which follows is presented from the point of view of the calling party's transmit path (which is the same as the called party's receive path), such discussion has applicability to the called party's transmit path/calling party's receive path, with for example, the roles of elements 32 and 33 reversed.)
The part of the network which is digital--that part between and including D/A-A/D converters 24, 25--exhibits no unintentional loss (there are -6 dB attenuators 28, 29, however, which are intentionally placed in the circuit). The analog part of the network--the balance of the network diagram of FIG. 1--does suffer unintentional loss, however. This loss is variable depending on the length of the local loop 31, 34 between the CPE 20, 21 and the central office. In addition, the level of a speech signal presented to the analog part of the network is variable, depending on the CPE (telephone) 20, 21 microphone efficiency, as well as how loudly a person is speaking into the microphone and how close the person's mouth is to the microphone. As shown in the Figure, the average loss on the analog portion of the calling party's transmit path--referred to as TOLR (telephone +local loop loss) is -46 dB.
As discussed in greater detail in the referenced patents, TrueVoice.RTM. 33 sound enhancement operates to mitigate the effect of signal loss in a telephone network connection for signals traveling from the calling party to the called party. TrueVoice 33 sits in the middle of the digital network and adds gain of a fixed amount (4 dB) to a computed input power of a transmitted speech signal. FIG. 2 illustrates this. The power of the transmitted (input) signal is computed over a time interval. For example, the signal may have an average power over the interval of -21 dB. TrueVoice will amplify the signal such that the signal will have an average power of 4 dB better (or -17 dB). If the average power of the input signal is -17 dB, the output power will be raised to -13 dB. Through its combination of pre-emphasis (base boost) and the AVC, TrueVoice 33 compensates some or all of the network path 31, 35 attenuation, as well as CPE 20 efficiency variation, to improve how speech carried over the telephone connection "sounds" to someone listening.
Although there is an optimal TrueVoice 33 output power level to which the network signal could be adjusted, TrueVoice employs a conservative boost of, e.g., a constant 4 dB, to compensate for attenuation suffered in the paths 31 and 35 of the network. Unfortunately, there are several variables related to the paths 34 and 36 of the network which affect the amount of signal loss a speech signal may suffer in transmission over a telephone circuit. For example, the attenuator 29 is not always present in a long distance connection. Network response variability is also caused by variation in local-loop 34 length carrying received signals and variability in the efficiency of CPE 21's electric-to-acoustic transduction. This response variability can cause, among other things, variability of objective loudness as perceived by telephone customers. Moreover, because TrueVoice.RTM. 33 applies a conservative gain mapping (4 dB, for example) when administering active volume control, called parties connected on long loops or loops that cause great attenuation may not be able to perceive all the benefits of TrueVoice.RTM. 33. Since TrueVoice 33 does not know what the loss will be on the paths 36, 34 of the network, it does not compensate for such loss and, in fact, provides a relatively conservative maximum gain because of this.